1. Field of the Invention
The present invention relates to the field of decoding a GPS satellite data, and more specifically, to the field of decoding a GPS satellite data having a low signal-to-noise ratio (SNR).
2. Discussion of the Prior Art
The data stream transmitted by GPS satellites is described in detail in “Global Positioning System: Theory and Applications”, Volume 1, pages 121-176, third printing, 1996. The GPS 50 bits/s (Baud) data stream includes an important information used by a GPS receiver to perform a satisfactory navigation, to realize a valuable time transfer function, to obtain a geodetic surveying, and/or to perform a set of satisfactory velocity measurements. These navigation data are uploaded to each satellite by the GPS Control Segment (CS) for later broadcast to the user.
More specifically, the 50 Baud GPS data stream includes (a) Ephemeris information including a detailed information about a particular satellite's orbit position and clock offset, (b) Almanac information including a less accurate information about all satellite orbits and clock offsets, and (c) a satellite health information, that is whether any particular satellite should be used by a GPS receiver to facilitate position calculation. The capability to receive all relevant GPS data from the combination of information obtained from the 50 Baud data stream is one of the aspects that are critical to the receiver's operation in being able to compute position, time and velocity.
In the prior art, in low SNRs applications (for example, in indoor applications) a typical GPS receiver fails to decode 50 Baud data. As a result, in the prior art, the satellites emanating a low SNR data could not be used in a navigation solution.
What is needed is to improve a GPS receiver capability to decode a GPS data stream having low SNR, thus enabling GPS receivers to use a low SNR GPS satellite in a navigation solution.